Date of Award
histone modifications, G9a methylation, G9a protein sequence, GLP protein sequence, B cell development
The size and complexity of eukaryotic genomes require that specific mechanisms exist for ensuring both the stability as well as the accessibility of DNA. One such mechanism is the association of DNA with histones to form chromatin, the physiological substrate of gene expression. An important means by which histones impact transcriptional activity is through site-specific enzymatic modification of the amino terminal histone â€œtailsâ€, which can alter the spectrum of chromatin-associated proteins and hence transcriptional states. Among the known modifications of histones, lysine methylation has been proposed to represent a relatively stable mark which might mediate stable activation or repression, depending upon the site modified. The immune system provides an ideal system in which to test the physiological functions of particular chromatin-modifying activities, since proper lymphocyte development and function requires integration of multiple signals, both cell autonomous and receptor-mediated, with complex DNA recombination reactions which are unique to lymphocytes. We have exploited these features to explore the possible functions of histone 3, lysine 3 (H3K9) methylation in the immune system through conditional inactivation of the H3K9-specific methyltransferases G9a and GLP. These studies demonstrate that G9a is essential for B cell development in the mouse, but is dispensable for T cell development. The defect in B lymphopoiesis in the absence of G9a is caused by a block in development at the pro-B cell stage, corresponding to the onset of immunoglobulin heavy chain recombination. The overall normal behavior of G9a-deficient peripheral B cells argues in favor of the specificity of this effect. Furthermore, through analysis of G9a and GLP protein sequences, we have identified a novel mechanism by which chromatin-modifying complexes can be regulated. We find that both G9a and GLP contain conserved H3K9-like motifs, on which the main biochemical features of H3K9 itself are recapitulated. Considering both the sequence and functional conservation between these sites and histones, we term these motifs in nonhistone proteins â€œhistone mimicsâ€. Our initial analysis indicates that many chromatinassociated proteins potentially contain H3K9-type histone mimics, and that this phenomenon is therefore likely to be a general one.
Sampath, Srihari Cidambi, "Studies on the Histone Methyltransferase G9a" (2006). Student Theses and Dissertations. 60.